This invention relates to a solid-state image pickup device for use in picking up images. Such a solid-state image pickup device is particularly useful in a television camera.
A conventional solid-state image pickup device has a plurality of parallel columns of photosensitive elements on a semiconductor substrate for accumulating electric charges as sensed charges in response to incident light. A plurality of vertical shift registers are arranged along one side of each column. In order to transfer the accumulated charges to the adjacent vertical shift registers, a transfer gate is arranged between each of the columns of the photosensitive elements and the adjacent vertical shift registers. The transfer gate transfers the accumulated charges from the photosensitive elements to the adjacent vertical shift register in each vertical blanking time period. A horizontal shift register is coupled to one end of each vertical shift register. For one picture field, the horizontal shift register serially outputs the transferred charges as an image signal in each horizontal scanning time period. The picture field is for making a single image of an object. The above-described image pickup device is well known in the art as a CCD (Charge Coupled Device) solid-state image pickup device.
As known in the art, an accumulating time period for the photosensitive elements varies according to television standards. According to the NTSC system, wherein a television signal has sixty fields per second, the accumulating time period is equal to 1/60 second. Such an accumulating time period is much too long for the solid-state image pickup device and is not suitable for picking up the image of a moving object because the image becomes obscure. Furthermore, the long picking up time generates an overflow of sensed charges accumulated in the photosensing element to generate a blooming phenomenon.
In order to achieve a high speed sensing and to prevent the blooming phenomenon of sensed charges, electronic shutter operation is well known. The electronic shutter operation can be realized by a drain circuit or a charge drain region coupled to another end of each vertical shift register. The drain circuit or the like is for draining, as disused charges, the excessive charges accumulated in the photosensitive elements. With regard to the electronic shutter operation, the disused charges are transferred to the vertical shift registers at a beginning of the vertical blanking time period. The vertical shift registers shift the transferred disused charges to the drain circuit or the charge drain region in response to a drain shift pulse signal having a first clock rate. During shift operation of the transferred disused charges, the photosensitive elements afresh accumulate the electric charges. Subsequently, the accumulated charges are transferred again to the vertical shift register on completion of the electronic shutter operation. In response to a shift pulse signal having a second clock rate, the vertical shift registers shift the transferred charges to the horizontal shift register. The first clock rate for transferring the disused charges must be higher than the second clock rate for transferring the signal charges for the following reason.
A time interval between completion of transfer of the disused charges and beginning of transfer of the accumulated charges is called an electronic shutter time period. For example, the electronic shutter time period is equal to 1/1000 second. The disused charges must be shifted to the drain circuit or the like within the electronic shutter time period.
As will later be described in detail, the vertical shift register can shift a restricted quantity of the transferred disused charges in quantity. This means that when a large quantity of charges are introduced into the vertical shift register, the charges overflow from a stage in the vertical shift register to other stage thereof to cause a blooming phenomenon. To prevent such charge overflow, the excessive electric charges accumulated in photosensitive elements are drained by the electronic shutter operation. In the conventional solid-state image pickup device, however, it is hard to effectively utilize the electronic shutter operation.